A silicon nitride film is important as a protection film or an insulation film for a semiconductor device, and thermal CVD method or plasma CVD method is employed for forming the silicon nitride film. For the thermal CVD method, silane (SiH4) gas and ammonia (NH3) gas are employed, and a silicon nitride film is deposited by chemical vapor deposition on a surface of a substrate using thermal decomposition reaction at the temperature of 750˜800° C., for example. On the other hand, for the plasma CVD method, also SiH4 gas and NH3 gas are employed as reaction gases, applying high frequency electric field to the reaction gases, and activating those reaction gases by applying the electric energy of the high frequency electric field, and a silicon nitride film is deposited by chemical vapor deposition on a surface of a substrate using plasma reaction at the lower temperature of about 300° C. The silicon nitride film formed by such method has the disadvantage such that crack and/or flaking is easily happened due to intrusion of water or impurities or the like.
As a process for forming a silicon nitride film which is hardly happened crack and/or flaking, a process is disclosed in a patent document 1, for example. The disclosed process employs halogen gas excited by plasma, applies etching process to a surface of a substrate under vacuum condition so as to remove impurities entirely which has remained on the substrate and to form uniform and fine irregularities on the surface of the substrate, so that coherency of the silicon nitride film and the substrate is improved and quality of the film is improved. Also, a technique is disclosed in a patent document 2. The disclosed technique provides a pre-processing chamber at a prior stage with respect to a film forming chamber, irradiates ECR (Electron Cyclotron Resonance) plasma to a surface of a substrate within the pre-processing chamber so as to remove moisture absorbed to the surface of the substrate and moisture included in the thin film formed on the substrate.
Patent document 1: Japanese Patent Laid-Open Gazette No. Tokukaihei 5-315251 (No. 1993-315251 A)
Patent document 2: Japanese Patent Laid-Open Gazette No. Tokukaihei 5-331618 (No. 1993-331618 A)